[LLVMdev] sign extensions, SCEVs, and wrap flags

[Preston Briggs]

On Tue, Sep 18, 2012 at 10:59 PM, Andrew Trick <atrick at apple.com> wrote:

>
> On Sep 18, 2012, at 8:21 PM, Preston Briggs <preston.briggs at gmail.com>
> wrote:
>
> Given the following SCEV,
>
> *(sext i32 {2,+,1}<nw><%for.body> to i64)*
>
>
> from the following C source,
>
> *void strong3(int *A, int *B, int n) {*
> *  for (int i = 0; i < n; i++) {*
> *    A[i + 2] = i;*
> *    ...*
> *  }*
> *}*
>
>
> Since the No-Wrap flag is set on the addrec, can't I safely rewrite it as
>
> *{2,+,1}<nw><%for.body>*
>
>
> If I can, why isn't the SCEV package simplifying things for me?
>
>
> The short answer is that SCEV is terrible at preserving NSW flags. I
> personally don't believe they belong in SCEV but the merits of making any
> design change here are dubious.
>
> To understand one example of SCEV dropping NSW, see createSCEV for
> Instruction::Add. Synopsis: your add is not a "basic induction variable" so
> its NSW flag does not bound the number of loop iterations. We only know
> that the add's original IR users expect NSW. There could be other IR adds
> with the same expression, but without the NSW flag. SCEV doesn't know
> anything about acyclic control flow or IR users, so it must drop the flags.
>
> I would try hard not to rely on NSW flags on arbitrary SCEVs. I would
> first find the phi or basic induction variable before checking the
> recurrence's NSW flag. Or, better yet, only rely on SCEVAddRec's NW (no
> self-wrap flag) rather than NSW. Notice that the NW is preserved in your
> add's recurrence!
>
> -Andy
>
>
OK.  I think...
Basically, I'm trying to understand how two subscripts relate to one
another. When I find sign and zero extensions, life gets confusing. In an
effort to keep life simple, I begin by walking though the expressions,
trying to eliminate extensions where it won't change the answer. For
example, I think

*(sext i32 {2,+,1}<nw><%for.body> to i64)
*

is the same as

*{2,+,1}<nw><%for.body>*

right?

Mechanically, when I see an sext over an addrec and the addrec has the NW
flag, then I can rewrite is as an addrec<nw> with the base and step
extended. In this case, the base and step are constants, which are
particularly easy.

On the other hand, if the addrec is missing the NW flag, I'd be making a
mistake.

In a similar vein, it seems plausible that I can rewrite a sign-extend over
an add (or multiply), as long as the add (multiply) has the NSW flag,
right? Same for zero-extend, over add with NUW flag.

Thanks,
Preston
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